I bought an old JVC microphone. It has Primo cardioid capsule, the same capsule as Nak300 - CP1. This capsule had released nut that hold the rear chamber with backplate. Probably someone has looked inside. So I carefully opened it too. (I assembled it back, capsule works, but I'll have to try it in concert).

It's electret capsule. Diaphragm is permanently charged, it is not energized by phantom power like in true condenser mics. Mic is receiving on backplate (gegenelektrode, I like this word) very weak electrical signals that is generated by the movement of charged diaphragm. Then the signal is brought to the JFET, where it is amplified.

Function of backplate and rear chambers is described by DSatz, see link above. Holes in this backplate have a special arrangement, dumping is seen in some holes too (Figure 4). Contact for an output electrical signal is on the backplate center from behind, partially visible wire through holes.

The entire capsule and the rear chamber is probably the same for cards and omni (Figure 6). The only difference is that the cards have rear chamber partially opened on back - see yellow seal (Figure 5). This seal is regulated by black plastic nut (Figure 6). I will try to tighten this nut in future and I will let know the effect.

The mic is JVC M-510. It is the same mic as Nak100, it is powered by 1.5V. I bought two sets with these mics on ebay. Each set has mic body, cardiod cap and short shotgun. You can use these caps with Nak300, i have tried it. Black cap is Nak, silver is JVC. I am looking forward to try shotguns outdoor in summer in Gutbucket configuration (shotgun in the center with spaced omnis). But the main purpose, why i bought these mics, is that I want to try to power them from Chris preamp CA900. I would like to use a capsule with attenuator only and connect it directly to preamp.

I was surprised my Nak300 are a lot noisier, but who knows they all are old. JVC has higher output too, it is probably because their higher output impedance (JVC 600 ohm, Nak300 200 ohm).